Add service_namespace attr to span metrics

[marctc]

The resource attribute service_namespace is missing for span metrics generated
from OTEL such as http_server_request_duration_seconds_count. This PR adds
that attribute to these metrics.

Fixes #975

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.04%. Comparing base (b51cc68) to head (df178b8).

Files	Patch %	Lines
pkg/internal/request/span_getters.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #980      +/-   ##
==========================================
- Coverage   66.05%   66.04%   -0.02%     
==========================================
  Files         134      134              
  Lines       10648    10650       +2     
==========================================
  Hits         7034     7034              
- Misses       3124     3126       +2     
  Partials      490      490              

Flag	Coverage Δ
oats-test	36.30% <0.00%> (+0.01%)	⬆️
unittests	50.38% <0.00%> (-0.01%)	⬇️

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